U.S. application Ser. No. 316,861, filed Oct. 30, 1981 in the names of Jean-Pierre Marechal and Richard O. Maschmeyer and entitled Process to Mold Precision Glass Articles, discloses a molding procedure for obtaining a given finished optical element (a lens, for example) by reshaping a glass body in contact with a mold, the glass being at a viscosity between about 10.sup.8 -10.sup.12 poises. The capability of molding glass bodies to very precise tolerances is of great economic advantage in that costly grinding and polishing are eliminated. This is especially true where the configuration of the desired product is not regular, for example, an aspheric lens. As is explained, it is most advantageous to employ glass compositions having molding temperatures as low as possible to prevent rapid deterioration of molds.
It will be highly desirable to develop glass compositions suitable for precision molding optical glasses exhibiting high refractive indices (n.sub.D =1.65-1.735), reduced dispersions (V.sub.D =38-58), low transition temperatures (T.sub.g &lt;500.degree. C.), low softening points (T.sub.L &lt;600.degree. C.), and satisfactory chemical durabilities.
Classic optical glasses demonstrating similar optical properties employed heretofore have generally exhibited transition temperatures (T.sub.g) and softening temperatures (T.sub.L) higher than 600.degree. C. and 700.degree. C., respectively. As conventionally classified, those glasses belong to families termed "Special Barium Flints" and "Special Barium Crowns". The basic components of those glasses are SiO.sub.2, B.sub.2 O.sub.3, BaO, and La.sub.2 O.sub.3.